The present invention relates to a reference position detection method of a vehicle rear wheel steering apparatus and, more particularly, to a reference position detection method for detecting a neutral position of a mechanical system in a vehicle rear wheel steering apparatus for controlling a rear wheel steering shaft for steering rear wheels by two systems, i.e., a mechanical system and a hydraulic system.
Conventionally, as disclosed in, e.g., Japanese Patent Laid-Open No. 1-273772, a vehicle rear wheel steering apparatus is known. This apparatus comprises an input shaft for inputting a steering angle of front wheels, an output rod member which strokes in the axial direction upon reception of an input from the input shaft so as to steer rear wheels, and a yoke assembly, coupled to and swingably supported by the output rod, for controlling a stroke displacement in accordance with its swing angle.
In this apparatus, the steering control amount of rear wheels is changed in accordance with the stroke displacement of the output rod member on the basis of the swing angle of the yoke assembly. For this reason, in order to precisely steer the rear wheels, a neutral angular position (zero phase) serving as the reference position of the swing movement of the yoke assembly, i.e., a position where the output rod member is not stroke-displaced even when the input shaft is rotated must be precisely determined.
However, in the conventional apparatus, the yoke assembly is swung to a position near the neutral angular position under visual observation, and the true neutral angular position (reference position) where the output rod member is not stroke-displaced even when the input shaft is rotated is detected by trials and errors.
When the neutral angular position of the yoke assembly is detected by trials and errors in this manner, a time required for detecting the true neutral angular position is prolonged due to trying errors, resulting in a troublesome detection operation of the neutral angular position of the yoke assembly.
In the above-described rear wheel steering apparatus, the control steering amount of the rear wheels is determined in accordance with the stroke displacement of the output rod member on the basis of a rotational angle of the input shaft in addition to the yoke assembly. For this reason, in order to precisely steer the rear wheels, the neutral angular position (zero phase) serving as a reference rotational position of the input shaft must also be precisely determined.
In the conventional apparatus, the input shaft is rotated in practice, and a position where the output rod member is not stroke-displaced even when the input shaft is rotated is detected by trials and errors. In this manner, the neutral angular position as the reference position is determined.
When the neutral angular position of the input shaft is detected by trials and errors in this manner, a time required for detecting the neutral angular position is prolonged due to trying errors, resulting in a troublesome detection operation of the neutral angular position (reference position) of the input shaft.
The vehicle rear wheel steering apparatus described in Japanese Patent Laid-Open No. 1-273772 described above comprises the mechanical system for displacing the rear wheel steering shaft for steering the rear wheels, and the hydraulic system for assisting the displacement of the rear wheel steering shaft. The mechanical system comprises the input shaft for inputting the steering angle of the front wheels, the output rod member which strokes in the axial direction upon reception of an input from the input shaft so as to steer the rear wheels, and the yoke assembly, coupled to and swingably supported by the output rod, for controlling a stroke amount of the output rod member upon rotation of the input shaft in accordance with its swing angle. The stroke amount of the output rod member is transmitted to the rear wheel steering shaft by a displacement transmission means coupled to the output rod member. The hydraulic system for assisting the displacement of the rear wheel steering shaft comprises a power steering means for hydraulically assisting the displacement of the rear wheel steering shaft, and a hydraulic pressure control valve for controlling the power steering means. The hydraulic pressure control valve is coupled to the displacement transmission means, so that the mechanical and hydraulic systems are connected to each other.
In the mechanical system of the rear wheel steering apparatus, the stroke amount of the output rod member, i.e., the displacement of the rear wheel steering shaft is determined in accordance with the rotational angle of the input shaft and the swing angle of the yoke assembly. In the hydraulic system, the rear wheel steering shaft is hydraulically assisted so as to be displaced by an amount corresponding to the determined displacement by the power steering means.
Therefore, in order to precisely steer the rear wheels, i.e., to precisely control the rear wheel steering shaft, high-precision neutral adjustment must be performed for both the mechanical and hydraulic systems, and thereafter, these systems must be connected so that the neutral positions (zero phases) of the two systems coincide with each other.
Of the two systems, when the neutral adjustment of the mechanical system, i.e., of the input shaft and the yoke assembly is to be performed, the neutral adjustment of, e.g., the input shaft can be performed as follows. That is, even when the yoke assembly is located at any angular position corresponding to the in-phase side or the opposite phase side, if the input shaft is pivoted over the entire pivoting range, it always passes the neutral angular position. Paying attention to this fact, while the yoke assembly is fixed in a swing angular position corresponding to a specific in-phase position, and in a swing angular position corresponding to a specific opposite phase position, the input shaft is pivoted once over the entire pivoting range for each of these positions. Changes in stroke of the output rod member in accordance with the rotational angle (input angle) of the input shaft in these pivoting operations are expressed as graphs. An intersection between the two graphs is determined as the neutral angular position, thereby detecting the neutral angular position of the input shaft.
However, the mechanical system of the rear wheel steering apparatus includes many meshing portions among gears, and many fitting portions among parts, and a hysteresis occurs in stroke characteristics of the output rod member with respect to the input angle due to cluttering of these portions and, flexure of parts themselves. For this reason, even when the two graphs are obtained, an intersection between these graphs cannot be determined as one point.
With the conventional method described above, although the neutral position can be determined to be within a given range, it is difficult to detect a precise neutral position.